1. Field of the Invention
The present invention relates to an electrophoretic display device wherein images or patterns are formed on the display device by electrophoretic migration of charged particles.
2. Related Background Art
The amount of information that can be handled by each individual is rapidly increasing due to the remarkable advancement in the digital technology. Along with this tendency, thin display devices with low power consumption as information output devices have been extensively developed.
As one of such display devices, an electrophoretic display device is described by Harold D. Lee et al in U.S. Pat. No. 3,612,758. FIG. 11 shows an example of a structure of the electrophoretic display device. The electrophoretic display device of this type is equipped with a pair of substrates 1 and 2 disposed opposite each other with a predetermined gap provided between the substrates 1 and 2, a dielectric liquid 3 filled in the gap between the substrates 1 and 2, numerous colored charged electrophoretic particles 4 dispersed in the dielectric liquid 3, and electrodes 14 and 15 disposed at respective pixels Λ along the substrates 1 and 2, respectively. In this device, the colored charged electrophoretic particles 4 are charged in a positive polarity or a negative polarity, such that the colored charged electrophoretic particles 4 are attracted to the electrodes 14 or to the electrodes 15 when a voltage is applied to the electrodes 14 or the electrodes 15 depending on the polarity of the voltage applied. Since the dielectric liquid 3 and the colored charged electrophoretic particles 4 are colored differently from one another, when the colored charged electrophoretic particles 4 are attracted to the electrodes 15 on a viewer's side, the color of the particles 4 can be visually recognized by the viewer (see FIG. 11(a)); and when the colored charged electrophoretic particles 4 are attracted to the electrodes 14 on the opposite side, the color of the dielectric liquid 3 can be visually recognized (see FIG. 11(b)). Accordingly, by controlling the polarity of the applied voltage for each of the pixels, various images can be displayed. Hereafter, the device of the type described above shall be referred to as a “vertical migration type.”
In the vertical migration type electrophoretic display device, the dielectric liquid needs to be colored by adding and mixing coloring agents such as pigments, ions or the like in the liquid. By so doing, transfer of charges occur due to the coloring agents, which adversely affects electrophoretic movements of the charged electrophoretic particles, and deteriorates the service life and stability of the display device. Also, when the dielectric liquid is colored, the pigments in the dielectric liquid are adsorbed in the charged electrophoretic particles, which results in a problem of lowered display contrast. Furthermore, the colored dielectric liquid may enter a gap between the colored charged electrophoretic particles 4 and the electrodes 15, which lowers the display contrast.
To solve the problems described above, an electrophoretic display device of the type shown in FIG. 12 has been proposed. The electrophoretic display device shown in FIG. 12 is equipped with a pair of first substrate and second substrate disposed opposite each other with a predetermined gap provided between them, a dielectric liquid filled in the gap between these substrates, numerous charged electrophoretic particles dispersed in the dielectric liquid, and a pair of first electrode 11 and second electrodes 12 disposed at each pixel. In this display device, the pair of first electrode 11 and second electrodes 12 are not disposed in such a way to sandwich the dielectric liquid like the vertical migration type electrophoretic display device described above. Instead, the first electrode 11 is disposed along the first substrate, and each of the second electrodes 12 is formed in area that is shielded by a shield layer 13 provided on the second substrate. Also, a colored layer in a color different from that of the charged electrophoretic particles is formed at the first electrode 11, or on its surface, or in the rear of the first electrode 11 (in this case, the first electrode is transparent). When a voltage is applied to the first and second electrodes, the charged electrophoretic particles migrate depending on the polarity of the applied voltage. When the charged electrophoretic particles are attracted to the second electrodes below the shield layers, the charged electrophoretic particles are blocked by the shield layers from the viewer, such that the color of the charged electrophoretic particles cannot be seen by the viewer, and only the color of the first electrode and the shield layers can be visually recognized. On the other hand, when the charged electrophoretic particles are attracted to the first electrode, the color of the charged electrophoretic particles and the shield layers can be visually recognized. Accordingly, by controlling the polarity of the applied voltage for each of the pixels, images can be displayed.
Since the dielectric liquid in the electrophoretic display device described above may be transparent and does not need to mix with coloring agents, the problems entailed by the vertical migration type electrophoretic display device can be avoided.
However, the electrophoretic display device described above has the following problems. The electrophoretic display device shown in FIG. 12 has a structure in which the first electrode is provided flat on the first substrate, and the second electrodes are provided below the shield layers provided around the first electrode. With this structure, when a voltage is applied to the first and second electrodes to migrate the charged electrophoretic particles, it is obvious from FIG. 12 that an electric filed generated is concentrated in areas of the surface of the first electrode which are closest to the second electrodes, in other words, at end sections of the first electrode. This means that the electric field that can migrate the charged electrophoretic particles is weaker in a central area of the surface of the first electrode, which is located far from the second electrodes. For this reason, if, for example, the charged electrophoretic particles on the second electrodes are to be migrated onto the first electrode, many of the charged electrophoretic particles converge on the end sections of the first electrode, and it is difficult for the charged electrophoretic particles to reach the central area of the first electrode. Also, once the charged electrophoretic particles have moved to the central area of the surface of the first electrode, then it is difficult to migrate the particles to the second electrodes, and the charged electrophoretic particles are fixedly adhered to the central area of the first electrode. As a result, this causes a major problem of lowered display contrast.
A driving voltage may be set sufficiently large so as to move the charged electrophoretic particles to the central area of the first electrode. However, by so doing, problems relating to the dielectric strength make the active matrix driving employing switching elements difficult.
Also, in view of the active matrix driving, the conventional electrophoretic display device has another problem. As shown in FIG. 12, there is almost no area in the electrophoretic display device where the first electrode and the second electrodes overlap each other. For this reason, the electric capacity of the display panel is extremely small. As a result, when the active matrix driving employing switching elements is to be performed, a sufficiently large auxiliary electric capacity needs to be additionally formed, and the degree of freedom in designing the switching elements becomes lowered.